No comment on your math, I think the other scientists covered it.
Next time you order primers, or if you have a master stock that you can dilute, take note of the amount of DNA (usually in nanomoles) that the company delivers to you. Calculate the volume of solvent (water or Tris pH 8) to add to make a 100 pmol per ul stock. I typically add 100 pmol of each primer (forward and reverse) per reaction, so I just add 1 ul of each primer to the reaction mix. Less calculation AND you don't have to pipet less than 1 ul.
For example, if the primer company delivers 350 nmol of primer (typical amount), add 3500 ul of buffer to make 0.1 nmol/ul or 100 pmol/ul.